Conductive particle trap for high-power, gas-insulated transmission system

ABSTRACT

A method of placing a low-frequency, high voltage, gas-filled power transmission system into service by applying a conditioning voltage to the system prior to application of system voltage.

United States Patent n91 Cronin Feb. 12,1974

[ 1 CONDUCTIVE PARTICLE TRAP FOR HIGH-POWER, GAS-INSULATED TRANSMISSIONSYSTEM [75] Inventor:

[73] Assignee: I.T.E. Imperial Corporation,

Philadelphia, Pa.

[22] Filed: Aug. 21, 1972 [21] Appl. No.: 282,366

John C. Cronin, Greensburg, Pa.

[52] US. Cl 174/28, 174/14 R, 317/3, 317/262 R [51] Int. Cl. H011) 9/06[58] Field of Search... 317/3; 174/14 R, 28; 55/146, 55/154 [56]References Cited UNITED STATES PATENTS 3,515,939 6/1970 Trump 317/33,433,883 3/1969 Hahne 174/28 FOREIGN PATENTS OR APPLICATIONS 1,133,27011/1968 Great Britain 174/28 Primary Examiner-William M. Shoop, Jr.Assistant Examiner-Harry E. Moose, Jr. Attorney, Agent, orFirm-Ostro1enk, Faber, Gerb &

Soffen [57] ABSTRACT A method of placing a low-frequency, high voltage,gas-filled power transmission system into service by applying aconditioning voltage to the system prior to application of systemvoltage.

4 Claims, 3 Drawing Figures CONDUCTIVE PARTICLE TRAP FOR HIGH-POWER,GAS-llNSUlLATED TRANSMISSION SYSTEM BACKGROUND OF THE INVENTION Thisinvention relates to high-power, gas-insulated transmission systems, andmore specifically relates to conductive particle traps for trappingconductive or semiconductive particles in a gas-filled transmissionsystem which is operated at from 50 to 60 Hz. and at voltages in excessof 60,000 volts.

In gas-insulated systems, it is essential to prevent conducting orsemiconducting particles from entering the gas space in between thesystem conductors where an appreciable electric field exists. Thus, itcan be demonstrated that very severe reductions in dielectric strengthoccur where conductive particles are present in a gas space.

Several techniques have been proposed in the past to control themovement of these particles.

By way of example, US. Pat. No. 3,515,939 to Trump discloses the use ofan essentially zero-field region within the gas space which acts to trapconducting particles. Thus, a perforated screen is placed within thegrounded outer housing of the gas-insulated transmission line.Conducting and semiconducting particles, which are contaminants in thegas space, pass through the holes of the screen and are unable toacquire enough energy in the low-field region defined between the screenand the outer housing to move back out through the screen.

Other methods proposed for the control of these particles within the gasspace employed the use of an adhesive or tacky material on the conductorsurfaces in order physically to hold particles which deposit on thecoating. This type arrangement is shown in US. Pat. No. 3,553,410.

In accordance with the present invention, a novel contamination controlstructure is provided which uses a low but not necessarily zerodielectric fields at the outer ground electrode. Thus, in the Trump US.Pat. No. 3,515,939, the attempt is made to produce an essentiallyzero-field region. in accordance with the present invention, however, alow-field region which contains insufficient energy to cause conductiveor semiconductive particles to move out of the field, against the forceof gravity, is provided by corrugating the outer housing.

When using an outer corrugated housing the electric field at the base orlargest diameter portion of the corrugations will be much lower than atthe surface of a smooth cylindrical enclosure with the same inner diameter as the smallest diameter portion of the corrugations. In anyspecific example, the reduction in field intensity will be a function ofthe system dimensions and in particular will depend upon the depth ofthe corrugations. In a preferred embodiment of the invention, thecorrugation depth is about one-tenth the outer diameter of thecorrugated housing so that only about onetenth of the dielectric stressexists at the inside surface of the corrugations, as compared to thestress which would exist at the surface of an equivalent smoothcylindrical enclosure of constant diameter. By varying the depth andshape of the corrugations, even greater reductions in stress can beachieved. However, no attempt is made to produce a field free region asin US. Pat. No. 3,515,939.

Corrugated housings per se have been used in the past in connection withelectrical coaxial conductors for high-frequency transmission lines.Thus, US. Pat. No. 3,433,883, shows a cable having a corrugated outerhousing. This cable, however, is for the transmission of electric powerat radio frequencies and relatively low voltages, as compared to the lowfrequency and high voltages used with the present invention and thecorrugations were used solely to allow the cable to flex if necessary.By contrast, the invention is used in relatively low-frequency powertransmission systems where this low frequency is the conventionalutility power frequency of from 50 to Hz. Moreover, the transmissionsystem of the invention is intended for transmission of extremely highvoltages (above 60,000 volts) which require the use of atmospheres suchas SP or gas mixtures including SF It is only when the voltages exceed,for example, 60,000 volts, that conductive and semiconductive particlesbecome a serious problem and jeopardize the dielectric integrity of thegas. Thus, the use of corrugated outer conductors in an air dielectricradio frequency transmission system is solely for cable flexibility, andis unrelated to a particle trapping function. This particle trappingfunction is realized for the first time with the new combination of theinvention of a corrugated outer housing in a gas-filled,power-transmission system which is operative at low frequency, and atpower line voltages in excess of 60,000 volts.

The efficiency of the corrugation as a particle trap in the presentinvention may be appreciated by considering that the ability ofparticles to move under the influence of an electric field is a functionof particle size and shape, particle density, gas density and theintensity of the electric field. Therefore, to move a particle whichrests at the base of the corrugations will require .a higher voltage onthe conductor than for a particle of similar size on the surface of asmooth enclosure. In the preferred embodiment of the invention, thisratio will be at least 10 to 1. Therefore, the gas-insulatedtransmission system will have its reliability improved since particleswhich will migrate in the bottom of the corrugations have a very lowprobability, essentially zero, of moving upwardly and into thehigh-stress regions.

In accordance with another aspect of the invention, before a system isput into service, a conditioning voltage may first be applied to allowmost particles capable of moving to migrate into the corrugations. Theinitial conditioning voltage is preferably obtained from a low energy orhigh impedance source such as a d-c test set and acts to move conductiveparticles created during installation or maintenance into thecorrugation bottoms. By using a high impedance or low energy source forthe conditioning voltage, flashover during the conditioning period willnot cause permanent damage. The system voltage may then be applied tothe line after this initial purging action.

The use of the corrugated housing of the invention has certain physicaladvantages that will be apparent. Thus, the conductive enclosure impartsmechanical flexibility to the system, which can be advantageous when thesystem is to be laid on a relatively irregular support surface, such asthe bottom of a ditch, or a seabed, or the like.

Another advantage is that the corrugations are formed in the enclosureduring its fabrication, to eliminate the need for adding additionalcomponents, such as perforated shields or adhesive surfaces duringassembly of the bus system.

A major advantage of the invention is that the corrugations produceparticle traps which run the entire length of the transmission system.This is in contrast to discrete, longitudinally spaced traps along thelength of the transmission line, where the probability of quicklytrapping particles after their generation is relatively low. Thus,particles which are generated during operation of the transmission linedue, for example, to sliding contacts between the insulator supports andbus will be immediately trapped with the present invention, since theydo not have to migrate along the length system until they reach a trap,as in the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinalcross-sectional drawing of a bus constructed in accordance with thepresent invention.

FIG. 2 is a cross-sectional view of FIG. 1 taken across the section line22 in FIG. 1.

FIG. 3 illustrates an embodiment of the invention, in which the bottomof the corrugations is formed with a relatively sharp angle to lower thefield strength at the bottom of the corrugations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS.1 and 2, the invention is illustrated as applied to a transmission linewhich connects a relatively low-frequency generating station 10, shownas a 50 to 60 Hz. source, which may have an output voltage in excess of60,000 volts and, for example, 230,000 volts. The transmission line 11of the invention then connects this source to a suitable load circuit,schematically illustrated as the load 12.

Obviously, the system may be multi-phase but only one of the phaseconductors is shown in FIG. 1. Thus, the conductor of FIG. 1 consists ofa central conductor 13 which is enclosed by a corrugated groundedconductive housing 14. Conductor 13 will be sized in accordance with thecurrent which must be carried by the transmission line, and isconventionally supported within the corrugated housing 14 by a pluralityof longitudinally spaced insulators, one of which is shown as insulatordisk 15. The interior of the enclosure is then filled with anelectro-negative gas, for example, sulfur hexafluoride, at a pressure of3 to 15 atmospheres. Suitable gas mixtures such as mixtures of SF 6 andnitrogen may be used to reduce gas liquification problems.

In one embodiment of the invention, the outer diameter D of thecorrugations may be 12 inches while the inner diameter of thecorrugations D may be 10.75 inches. The distance between the peaks ofadjacent corrugations, shown in FIG. 1, as distance D may typically be3.5 inches. The diameter of the conductor 13 may typically be 4.5inches.

The system is then capable of transmitting low frequency power of from50 to 60 Hz. at a voltage of 230 KV, by virtue of the careful control ofthe construction of insulators 15 and by virtue of the dielectricproperties of the insulation gas such as the pressurized sulfurhexafluoride gas within housing 14.

In accordance with the invention, the corrugated housing in alow-frequency, high-voltage, gas-filled power transmission system,causes a plurality of continuous particle traps to be defined along thelength of the system by virtue of the low-field intensity at the outerdiameter regions of the internal corrugated surfaces as compared to thefield intensity of regions within housing 14 which are closer toconductor 13. Thus, at the bottom of the corrugations (it being notedthat housing 14 will be horizontally disposed), the electric field atsurface 14a will be one-tenth the field at some point intermediate thehousing 14 and conductor 13. Moreover, this relatively low-field regionwill not be able to impart sufficient energy to contamination particleswhich are conductive or semiconductive and which are formed or existwithin the housing 14, so that such particles will be trapped by gravityat the bottoms of the corrugation regions.

Note that the trap exists completely along the length of thetransmission system when a corrugated housing 14 is used, so thatparticles need not have any appreciable longitudinal migration beforebeing trapped.

It will be further observed that the power transmission system will berelatively flexible by virtue of the corrugated outer housing 14, ascompared to a rigid system which would employ a cylindrical outerhousing having a constant diameter.

In placing this system in operation, it may be preferable to initiallyoperate the system at a voltage lower than rated voltage before applyingload to the transmission line. This will initially trap conductive andsemiconductive contaminants which were produced during the installationof the line. Thus, an initial voltage above 60,000 volts is applied tothe line before connection to a load circuit, causing the trapping ofmost conductive particles in the low-field regionsin the corrugations.The rated voltage, for example, 230,000 volts may thereafter be appliedand the load circuits connected with the gas region within thetransmission line substantially free of conductive particle contaminantscreated during installation.

In FIG. 1, the corrugations in housing 14 are formed so that thecorrugations are generally sinusoidal in cross-section. FIG. 3 shows asecond embodiment of the invention, wherein the corrugations 20 areformed with generally rounded inner diameter sections 21 and relativelysharp outer diameter sections 22. Thus, the area within outer diametersections 22 will have a lower field intensity than they would if thecorrugations were sinusoidally shaped and, therefore, serve as improvedparticle trapping regions. Note that the corrugation shape could also beV shaped in cross-section or could have any other desired section forproducing a desired low-field intensity at the corrugation bottom.

Although the present invention has been described in connection with apreferred embodiment thereof, many variations and modifications will nowbecome apparent to those skilled in the art. It is preferred, therefore,that the present invention be limited not by the specific disclosureherein, but only by the appended claims.

The embodiments of the invention in which an exclusive privilege orproperty is claimed are defined as follows:

1. The method of placing a low-frequency, high voltage, gas-filled powertransmission system into service, said power transmission systemincluding a horizontally disposed elongated central conductor surroundedby and insulated from an outer corrugated housing, with the spacebetween the said central conductor and said corrugated housing beingfilled by an insulation gas; said method comprising the steps ofinitially applying a conditioning voltage between said outer housing andsaid central conductor from a relatively low energy conditioning sourcefor a length of time sufficient to cause substantially all conductingand semiconducting particles which contaminate said gas to move tobottom regions of said corrugated housing and thereafter connectingsystem voltage to said power transmission system wherein the energy ofsaid system is substantially greater than the energy of saidconditioning source.

2. The method of placing a low-frequency, high voltage, gas-filled powertransmission system into service, said power transmission systemincluding a horizontally disposed elongated central conductor surroundedby and insulated from an outer housing which has a conductive particletrap means therein, with the space between the said central conductorand said housing being filled by an insulation gas; said methodcomprising the steps of initially applying a conditioning voltagebetween said outer housing and said central conductor from a relativelylow energy conditioning source for a length of time sufficient to causesubstantially all conducting and semiconducting particles whichcontaminate said gas to move to said conductive particle trap means ofsaid housing and thereafter connecting system voltage to said powertransmission system wherein the energy of said system is substantiallygreater than the energy of said conditioning source.

3. The method of placing a low-frequency, high voltage, gas-filled powertransmission system into service, said power transmission systemincluding a horizontally disposed elongated central conductor surroundedby and insulated from an outer corrugated housing, with the spacebetween the said central conductor and said corrugated housing beingfilled by an insulation gas; said method comprising the steps ofinitially applying a relatively low voltage between said outer housingand said central conductor for a length of time sufficient to causesubstantially all conducting and semiconducting particles whichcontaminate said gas to move to bottom regions of said corrugatedhousing and thereafter connecting system voltage to said powertransmission system.

4. The method of placing 'a low-frequency, high voltage, gas-filledpower transmission system into service, said power transmission systemincluding a horizontally disposed elongated central conductor surroundedby and insulated from an outer housing which has a conductive particletrap means therein, with the space between the said central conductorand said housing being filled by an insulation gas; said methodcomprising the steps of initially applying a relatively low voltagebetween said outer housing and said central conductor for a length oftime sufficient to cause substantially all conducting and semiconductingparticles which contaminate said gas to move to said conductive particletrap means of said housing and thereafter connecting system voltage tosaid power transmission system.

1. The method of placing a low-frequency, high voltage, gasfilled powertransmission system into service, said power transmission systemincluding a horizontally disposed elongated central conductor surroundedby and insulated from an outer corrugated housing, with the spacebetween the said central conductor and said corrugated housing beingfilled by an insulation gas; said method comprising the steps ofinitially applying a conditioning voltage between said outer housing andsaid central conductor from a relatively low energy conditioning sourcefor a length of time sufficient to cause substantially all conductingand semiconducting particles which contaminate said gas to move tobottom regions of said corrugated housing and thereafter connectingsystem voltage to said power transmission system wherein the energy ofsaid system is substantially greater than the energy of saidconditioning source.
 2. The method of placing a low-frequency, highvoltage, gas-filled power transmission system into service, said powertransmission system including a horizontally disposed elongated centralconductor surrounded by and insulated from an outer housing which has aconductive particle trap means therein, with the space between the saidcentral conductor and said housing being filled by an insulation gas;said method comprising the steps of initially applying a conditioningvoltage between said outer housing and said central conductor from arelatively low energy conditioning source for a length of timesufficient to cause substantially all conducting and semiconductingparticles which contaminate said gas to move to said conductive particletrap means of said housing and thereafter connecting system voltage tosaid power transmission system wherein the energy of said system issubstantially greater than the energy of said conditioning source. 3.The method of placinG a low-frequency, high voltage, gas-filled powertransmission system into service, said power transmission systemincluding a horizontally disposed elongated central conductor surroundedby and insulated from an outer corrugated housing, with the spacebetween the said central conductor and said corrugated housing beingfilled by an insulation gas; said method comprising the steps ofinitially applying a relatively low voltage between said outer housingand said central conductor for a length of time sufficient to causesubstantially all conducting and semiconducting particles whichcontaminate said gas to move to bottom regions of said corrugatedhousing and thereafter connecting system voltage to said powertransmission system.
 4. The method of placing a low-frequency, highvoltage, gas-filled power transmission system into service, said powertransmission system including a horizontally disposed elongated centralconductor surrounded by and insulated from an outer housing which has aconductive particle trap means therein, with the space between the saidcentral conductor and said housing being filled by an insulation gas;said method comprising the steps of initially applying a relatively lowvoltage between said outer housing and said central conductor for alength of time sufficient to cause substantially all conducting andsemiconducting particles which contaminate said gas to move to saidconductive particle trap means of said housing and thereafter connectingsystem voltage to said power transmission system.